Composition and process for manufacturing PBT catheter balloons

ABSTRACT

A processing technique and a composition which allow compositions of PBT polymers or copolymers to be formed into blow molded articles, especially medical balloons such as dilatation or stent placement balloons, from extruded tubular parisons. The process includes a longitudinal stretch step run at a temperature below the Tg of the polymeric material and a radial expansion step run at a temperature above the Tg of the polymeric material. During the longitudinal stretch step, the tubing is subjected to high internal pressure. In the composition a small amount of boric acid is added to the polybutylene terephthalate in a melt blend to give a formulation which has reduced crystallinity after extrusion and which can more readily fashioned into high strength blow molded articles than PBT by itself. Articles, which may be formed using the invention include medical device balloon articles in which the polymer consists essentially of PBT optionally blended with boric acid.

This is a continuation of application Ser. No. 10/179,646 filed Jun. 24,2002, now U.S. Pat. No. 6,787,095, which is a divisional of applicationSer. No. 09/706,266, filed Nov. 30, 2000, now U.S. Pat. No. 6,465,067,which is a continuation of Ser. No. 09/657,494, filed Sep. 8, 2000, nowabandoned, which is a division of Ser. No. 09/034,431, filed Mar. 4,1998, now abandoned, all incorporated herein by reference.

BACKGROUND OF THE INVENTION

Devices having a balloon mounted at the distal end of a catheter areuseful in a variety of medical procedures. A balloon reservoir may beused to deliver a biologically compatible fluid, such a radiologicallyopaque fluid for contrast x-rays, to a site within the body. Radialexpansion of a balloon may be used to expand or inflate a stentpositioned within the body. A balloon may also be used to widen a vesselinto which the catheter is inserted by dilating the blocked vessel. Forexample, in the technique of balloon angioplasty, a catheter is insertedfor long distances into blood vessels of extremely reduced diameter andused to release or dilate stenoses therein by balloon inflation. Theseapplications require extremely thin walled high strength relativelyinelastic balloons of accurately predictable inflation properties.

Dilatation balloons made from PET (polyethylene terephthalate) are wellknown and widely used for angioplasty, stent placement, treatments inthe gastrointestinal, urethral, or reproductive tracts, and for othermedical purposes. Other polymer materials have also been reported to beuseful in such applications and some of those polymer materials havealso been used commercially, for instance, polyethylene, polyvinylchloride, Surlyn® polyethylene ionomer copolymer, nylon 12 and Pebax®polyamide-polyether-polyester block copolymer. In a number of referencespertaining to the formation of dilatation balloons, PBT (polybutyleneterephthalate) is mentioned as a suitable balloon material, alone or asone layer of a laminate balloon. Such statements may be found in EP 0745 395 A2 (Ethicon); U.S. Pat. No. 5,270,086 (Hamlin); and U.S. Pat.No. 5,304,340 (Downey). To date, however, no reference has actuallyreported the preparation of a PBT dilatation balloon, or even a PBTballoon layer.

Balloons made from poly(butyleneterephthalate)-block-poly(tetramethylene oxide) are described in U.S.Pat. No. 5,556,383 (L. Wang, et al), incorporated herein by reference.In U.S. Pat. No. 5,316,016 there is described a diagnostic imagingballoon for use on catheters to obtain an image of the configuration ofan internal lesion or other body structure. The balloon has a memoryeffect when inflated with a pressure in a certain low pressure range.The polymer material used to prepare the balloon is a blend of PBT and anon-crystallizing ethylene/cyclohexanedimethylene terephthalatecopolyester. This document states that it may be possible to preparesuch balloons using PBT alone but to date no such balloons have beenprepared. The imaging balloons prepared according to this patent are notsuitable for dilatation or for other high pressure applications.

A major reason why PBT has not been used to make such balloons is theextremely high crystallization rate which the polymer displays. The highcrystallization rate of polybutylene terephthalate makes it especiallysuitable as a molding resin where rapid crystallization reduces moldresidence time. However for articles which are prepared from an extrudedparison by a blow molding process, use of PBT polymer material hasproven to be extremely difficult or impossible. This is because evenvery rapidly quenched extrusions are typically so high in crystallinitythat the parison cannot be further processed in a practical manner.Opacification of the parison, another effect of PBT's highcrystallinity, may also have been perceived as a problem for qualitycontrol in a manufacturing process. References describing thecrystallization rate of PBT and/or its impact on thermoformingapplications include: M. Gilbert, et al., “Effect of Chemical Structureon Crystallization Rates and Melting of Polymers: Part 1. AromaticPolyesters,” Polymer, 13, 327-332 (July 1972); E. Chang, et al., “TheEffect of Additives on the Crystallization of Poly(ButyleneTerephthalate),” Polymer Engineering and Science, 18, 932-936 (September1978); U.S. Pat. No. 5,213,754, Kawaguchi, et al. (May 1993); and U.S.Pat. No. 5,128,404, Howe (July 1992).

In U.S. Pat. No. 5,213,754 there is described a polyester containerprepared from a melt molded film of a butylene terephthalatecopolyester. The copolyester is prepared from terephthalic acid,1,4-butane diol and an alkylene oxide adduct of a bisphenol compound.The copolyester is used to provide a lower crystallization rate comparedto PBT homopolymer material. The lower crystallization rate takentogether with specific subsequent processing steps allows athermoformable sheet to be obtained.

In U.S. Pat. No. 5,128,404 blow moldable PBT blend compositions areprepared by melt blending a mixture consisting essentially of PBT, anethylene copolymer containing epoxide groups and an ionomer obtained byneutralizing a (meth)acrylic acid functional polymer with Na⁺ or K⁺.

In U.S. Pat. No. 4,380,621 crystallization rate of alkaline carboxylateterminated PET is reported to have been increased by use of boric acidas a polymer additive or of sodium borate as a polymerization additive.

It would be desirable to be able to make medical balloons such asdilatation or stent placement balloons out of PBT because the materialoffers the potential of achieving strength properties similar to PET butwith better rewrap and lesion crossing characteristics. There thereforeexists a need for improved processing techniques or for improvedformulations which allow formation of high strength PBT balloons.

SUMMARY OF THE INVENTION

The present invention relates, in one aspect, to a processing techniquewhich allows compositions of PBT polymers or copolymers to be formedinto blow molded articles, especially medical balloons such asdilatation or stent placement balloons, from extruded tubular parisons.The process includes a longitudinal necking step run at a temperature ator below the Tg of the polymeric material and a radial expansion steprun at a temperature above the Tg of the polymeric material. Acharacterizing feature of the inventive process is that, during thelongitudinal necking step, the tubing is subjected to high internalpressure.

In another aspect the present invention relates to novel PBT-containingformulations which provide improved blow molding processability whileproducing a high strength formed article. According to this aspect ofthe invention, it has been discovered that a small amount of boric acidadded to the polybutylene terephthalate in a melt blend gives aformulation which has reduced crystallinity after extrusion, asevidenced by improved extrusion clarity, and which can more readilyfashioned into high strength blow molded articles than PBT by itself.

While the processing and formulation aspects of the invention can bepracticed independently of each other, in preferred embodiments of theinvention they are practiced together.

The invention is also directed to articles, especially medical deviceballoon articles, formed by blow molding of a polymer composition inwhich the polymer consists essentially of PBT. Articles formed using theprocess of the invention and/or the PBT/boric acid formulation describedherein are also within the scope of the present invention.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a perspective fragmentary view of a balloon catheter having aballoon thereon made in accordance with the invention.

FIG. 2 is a side sectional view of a balloon in accordance with oneembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION Polymers

The polymer materials which may be used in the invention have a highbutylene terephthalate content. Such materials often displayopacification when extruded and/or cannot be successfully blown afterextrusion into a tubular parison. They include:

-   -   a) poly(butylene terephthalate) homopolymer,    -   b) random polyester copolymers having above 80% butylene        terephthalate repeat units,    -   c) block copolymers comprising about 60% or more by weight of        poly(butylene terephthalate),    -   d) mixtures of at least two of a), b) and/or c); and    -   e) mixtures of one or more of a), b) and/or c) with no more than        10% by weight of another polymer.

Examples of PBT homopolymers include Celanex 1600A, Celanex 1700A, bothsold by Hoechst Celanese Corporation, and Ultradur B4500, and KR 4036,both PBT polymers sold by BASF. Polyester random copolymers with highbutylene terephthalate content which also form whitened parisons onextrusion may also be used beneficially in the inventive process. Suchcopolymers will typically have at least 90% butylene terephthalatemonomer units, more typically about 95% or more, and preferably at least98%, butylene terephthalate monomer units. Block copolymers having PBTblocks constituting about 60% by weight or more, for instancePBT-polybutylene oxide block copolymers, may also be employed. Suchblock copolymers include segmented block copolymers sold under theHytrel® and Arnitel® trademarks. Blends of PBT with up to about 10%,preferably no more than 5%, and more preferably no more than about 2%,of another polymer may also be usefully employed. Examples of polymerswhich can be included in such blends include other polyesters, such asPET; polyurethanes, especially polyurethanes derived from polyesterpolyols; polycarbonates, poly(meth)acrylates and maleate polymers.

Process

For catheter balloons, extrusions may be prepared according toconventional procedures used for other polymer materials such as PET,nylon or Pebax®.

The tube is then stressed in the axial direction at a temperature wherenecking occurs when axial stretching force is applied. Stressing in thismanner starts a elongation of the tube from a specific point along itslength where the tubing material is yieldingly pulled out until aspecific diameter is reached, at which time the necking stops andmaterial starts pulling successively from the adjacent unnecked region.In this way the point where the necking takes place propagates along thelength of the tubing until the a sufficient length of the tubing segmentto form a balloon has been necked down. Necking occurs when the materialis stretched at a temperature which is about the Tg of the material, orlower. For a block copolymer the Tg referred to here is the highest Tg.At temperatures higher than about the Tg temperature, the materialstretches uniformly, rather than by necking, when axially stressed.Typical necking temperatures will be in the range of 15-35° C., withambient room temperature of about 20-25° C. usually being acceptable.The elongation produced by this process is a function of tubing wallthickness. For medical catheter balloons suited for dilatation or stentplacement applications with nominal diameters of about 1.5-5.0 mm,extruded wall thicknesses in the range of 0.003-0.015 inch (0.076-0.38mm) and outer diameters in the range of 0.015-0.055 inch (0.38-1.4 mm)will typically be suitable.

In accordance with the process of the present invention, the tubing ispressurized during the necking step. The pressure should be high, butnot so high as to cause the parison to expand radially or burst duringthe stretch step. A typical pressure range for PBT homopolymer is fromabout 50 psi (345 kPa) to about 800 psi (5516 kPa), depending on wallthickness. Greater wall thickness will require greater pressure. Fordilatation balloon extrusions suitable pressures will typically be inthe range of from about 300 psi (2068 kPa) to about 500 psi (3447 kPa).Somewhat lower internal pressures during axial stretch may givebeneficial results for copolymers or PBT polymer blends.

Following the longitudinal stretch step, the tubing is blown into anarticle such as a medical catheter balloon. Free blowing may be used,but typically a mold will be employed. Suitable radial expansiontemperatures will usually be in the range of about 85-140° C., althoughin some cases temperatures as high as about 200° C. may be feasible. Formedical catheter balloons internal pressure of about 250 (1724 kPa) toabout 500 psi (3447 kPa) will generally be used to blow the balloon.

The two step process may be carried out in rapid sequence, for instanceby inserting extruded tubing into a heated bath, promptly pressurizingand stressing to neck down the tubing before the tubing temperatureexceeds the Tg of the material, and then blowing the balloon as thenecked tubing segment nears or reaches the bath temperature. In somesuch cases it may be possible to maintain a constant pressurization ofthe tubing during the necking step, during the interval between neckingand blowing, and during the blowing step.

Although the PBT polymer materials used in the present invention providea balloon with a high rate of crystallization even when rapidly quenchedfollowing the blowing step, if desired, further crystallization can beaccomplished by a heat setting step run at a temperature higher than theblow temperature (typically 5°-25° C. higher) but at a pressure lowerthan the blowing pressure (typically 30 psi (207 kPa) to about 100 psi(689 kPa). Heat setting can reduce balloon compliance and can increasethe burst pressure of the balloon. Heat setting procedures which may beadapted for use in the inventive process are described in EP 274 411 A2(C. R. Bard) and EP 592 885 A2 (C. R. Bard), both of which areincorporated herein by reference.

If it is desired to increase compliance or to provide a steppedcompliance profile, the blown balloon may be shrunk by heating to atemperature somewhat below the blowing temperature (suitably to about70° C.-80° C.) while pressurizing to at about 30 psi (207 kPa) to about100 psi (689 kPa). Shrinking procedures which may be adapted for use inthe inventive process are described in U.S. Pat. No. 5,348,538 (L. Wang,et al) and in WO 97/32624, both of which are incorporated herein byreference.

Composition

Compositions of high butylene terephthalate content polymers whichprovide improved blow moldability from an extruded parison are alsoprovided by the present invention. The compositions may be obtained bymelt blending the polymer material with 0.01% to 5.0%, preferably fromabout 0.05 to about 0.5%, and more preferably from about 0.1% to about0.3%, by weight of boric acid. The blend may be made in the extrusionmelter or in a premelt prepared prior to extrusion. Suitable polymermaterials are as described above. PBT homopolymer is preferred.

As boric acid is well tolerated by the body, and PBT is available infood contact grades, the use of the compositions of the invention tomake medical devices, or packaging articles for food, cosmetic orpharmaceutical applications is not seen to raise new biocompatibilityquestions.

Articles

Both the process and the compositions of the invention provide benefitsin obtaining high strength articles from PBT-based polymeric materials.The composition can be used to obtain blow molded articles by directlyblowing an extruded tubular parison without any prestretching of thetubing. Conversely, the pressurized prestretch step has beendemonstrated to allow balloon formation even in the absence of the boricacid additive of the inventive composition. However, when the process ofthe invention is practiced with a boric acid-containing composition ofthe invention, high strength products can be obtained at lower blowpressures than are required to form an article from a boric acid-freecomposition. The process and/or composition described above may beutilized to produce any kind of blow molded article for which it may bedesirable to use PBT homopolymer or other high butylene terephthalatecontent polymer. Medical catheter balloons of diameters from about1.25-40 mm, especially those in the range of about 1.5 to about 8 mm,are preferred articles to which the invention may be applied.

The balloons of the invention may be either single layer balloons, ormultilayer balloons in which at least one layer is a high butyleneterephthalate content polymer material as described above. The preferred1.5-8 mm diameter dilatation balloons of this invention are suitablyformed to provide a double wall thickness, measured on the uninflatedcollapsed balloon, of about 0.0004″-0.0025″.

Multilayer Balloons

Known techniques for producing multilayer balloons may be readilymodified to utilize a composition of the invention or the pressurizedstretch step to thereby enable the employment of the PBT layer material.Various techniques are known for producing such multilayer structures,including coextrusion as described in U.S. Pat. No. 5,195,969 (J. Wang,et al.); U.S. Pat. No. 5,290,306 (Trotta et al); and U.S. Pat. No.5,270,086 (Hamlin), and tube-in-tube techniques as described incopending U.S. application Ser. No. 08/611,664, filed 6 Mar. 1996; U.S.Pat. No. 5,512,051 (J. Wang, et al); and in WO 96/04951 (SchneiderInc.), all incorporated herein by reference.

In such multilayer balloons different high butylene terephthalatecontent polymeric materials may be employed as different layers of thesame balloon, one or all of which may employ the boric acid additive.For instance, a multilayer laminate balloon may be provided from acoextruded tube having an inner layer of a PBT homopolymer/boric acidcomposition and an outer layer of a compatible poly(ester-block-ether)material which gives improved puncture resistance and/or a softer, lessscratchy feel to give reduced vessel trauma in use.

Referring to FIG. 1 there is shown a catheter 10 comprising an elongatedtube 12 with a balloon 14, made of a layer of PBT treated with boricacid in accordance with the invention hereof, mounted at the distal endthereof.

Referring to FIG. 2 there is shown a catheter balloon 20 comprising aninner layer 22 of a PBT treated with boric acid as described herein, andan outer layer 24 of a relatively softer polymer such as apoly(ester-block-ether).

In addition to having one or more structural polymer layers, the balloonmay be provided with a nonstructural coating layer, for instance acoating of a lubricious polymer or of a antithrombotic material, toimprove surface properties of the balloon.

Those skilled in the art will recognize that other techniques known forpreparing blow molded articles can be readily modified in accordancewith the teachings and observations provided herein, and without undueexperimentation, to produce blow molded articles of high PBT contentpolymers according to the present invention.

The invention is illustrated by the following non-limiting examples.

EXAMPLES

Attempts to form balloons were made using a number of differenttechniques and different polyester materials as shown in Table 1 below.The “yes” entries in Table 1 indicate that a balloon was formed. The“no” entries indicate that the technique did not produce a balloon.Examples 1-9 illustrate the procedures used.

Example 1 Comparative—PET Balloon (Stretched Above Its Glass TransitionTemperature)

PET tubing of ID 0.0171 and OD 0.0310 inch (made from Traytuf 7357,Shell Chemical) was stretched uniformly at 2.25 stretch ratio at 90° C.The stretched tube was inserted into a 3 mm balloon mold and the balloonwas formed at 95° C. with a molding pressure of 300 psi and tension of 5grams. The balloon burst at 230 psi with average balloon wall thicknessof 0.00029 inch (double wall thickness was 0.00058). The distention from6 atm to 12 atm was 5.8%.

Example 2 Comparative—PET Balloon (Without Stretch)

The same PET tubing (unstretched) used in Example 1 was inserted into a3 mm balloon mold without stretching and a balloon was formed at 95° C.with the molding pressure of 180 psi (1241 kPa) and the tension of 20grams. The balloon burst at 283 psi (1951 kPa) with average balloon wallthickness of 0.000445 inch (0.0113 mm) (double wall thickness was0.00089 inch (0.0226 mm)). The distention from 6 atm to 12 atm (608-1216kPa) was 3.4%.

Example 3 Comparative—PTT Balloon (Without Stretch)

PTT (poly(trimethylene terephthalate) Sam Yang Co., Korea) tubing of ID0.0170 and OD 0.0370 inch was used to form a 3 mm balloon at 95° C. withthe molding pressure of 170 psi and the tension of 5 grams. The balloonburst at 232 psi with average balloon wall thickness of 0.00068 inch(double wall thickness was 0.00136). The distention from 6 atm to 12 atm(608-1216 kPa) was 9.6%.

Example 4 PBT Balloon (Without Stretch, Thinner Wall Tubing)

PBT tubing of ID 0.0200 and OD 0.0300 inch (made from Celanex PET 1600A,Hoechst Celanese) was used to form a 3 mm balloon at 95° C. with themolding pressure of 250 psi. The balloon burst at 162 psi with averageballoon wall thickness of 0.000315 inch (double wall thickness was0.00063). The distention of 6/10 atm was 8.4%.

Example 5 PBT Balloon (Without Stretch, Boric Acid Modified, ThinnerWall Tubing)

Boric acid, 0.2% by weight, was added into PBT resin (Celanex PET 1600A,Hoechst Celanese) during tubing extrusion. The extruded tubing with ID0.0200 and OD 0.0300 inch was used to form a 3 mm balloon at 95° C. withthe molding pressure of 250 psi. The balloon burst at 169 psi with anaverage balloon wall thickness of 0.00030 inch (double wall thicknesswas 0.00060). The distention of 6/10 atm was 9.4%.

Example 6 PBT Balloon (Room Temperature Stretch Without Pressure, BoricAcid Modified, Thinner Wall Tubing)

The same tubing as in example 5 was used. The tubing of ID 0.0200 and OD0.0300 inch was necked first at room temperature and then the neckedportion was inserted into a 3 mm balloon mold. A balloon was formed at95° C. with the molding pressure of 300 psi. The balloon burst at 184psi with average balloon wall thickness of 0.000295 inch (double wallthickness was 0.00059). The distention of 6/10 atm was 9.7%.

Example 7 PBT Balloon (Stretch With Pressure, Regular Tubing WallThickness)

PBT tubing of ID 0.0170 and OD 0.0350 inch (made from Celanex PBT 1600A,Hoechst Celanese) was necked with 500 psi pressure inside. Then thenecked portion was used to form a 3 mm balloon at 95° C. with themolding pressure of 320 psi and the tension of 5 grams. The balloonburst at 390 psi with average balloon wall thickness of 0.00067 inch(double wall thickness was 0.00134 inch). The distention from 6 atm to12 atm (608-1216 kPa) was 3.9%.

Example 8 PBT Balloon (Stretch With Pressure, Boric Acid Modified,Regular Tubing Wall Thickness)

PBT tubing ID 0.0170 and OD 0.0350 inch (made from Celanex PBT 1600A,Hoechst Celanese, with 0.2% boric acid by weight introduced in extrusionprocess) was necked with pressure 400 psi inside tubing at roomtemperature. Then the necked portion was used to form a 3 mm balloon at95° C. with the molding pressure of 390 psi and the tension of 5 grams.The balloon burst at 353 psi with average balloon wall thickness of0.000675 inch (double wall thickness was 0.00135). The distention from 6atm to 12 atm (608-1216 kPa) was 3.4%.

Example 9 PBT Balloon (Stretch With Pressure, Regular Tubing WallThickness)

PBT tubing of ID 0.0170 and OD 0.0350 inch (made from Ultradur B 4500,BASF) was necked with pressure 400 psi inside tubing at roomtemperature. Then the necked portion was used to form a 3 mm balloon at95° C. with the molding pressure of 400 psi and the tension of 5 grams.The balloon burst at 364 psi with average balloon wall thickness of0.000725 inch (double wall thickness was 0.00145). The distention from 6atm to 12 atm (608-1216 kPa) was 3.6%. TABLE 1 Effect of DifferentPolyester Balloon Forming Techniques and Compositions on BalloonFormation Materials (tubing size) PBT PBT 1600A PBT PBT PBT 1600AStretch Condition Molding PET PTT 1600A Boric acid B-4500 1600A Boricacid Illustrative Temp Pressure Pressure (0.0171 × (0.0170 × (0.0171 ×(0.0171 × (0.0171 × (0.0200 × (0.0200 × Example ° C. psi psi 0.0310)(0.0350) 0.0350) 0.0350) 0.0350) 0.0300) 0.0300) 1 90 — <300 yes no nono no no no 2, 3 none — <200 yes yes no no no 4, 5 none — 200-300 no nono yes yes none — 400-500 no no no 6 22 — 200-300 no no no no no no yes22 — 400-500 no no no no no no 22 200-300 200-500 no no no no no 22 400200-300 no no no no no 8 22 400 ˜400 no no no yes yes 22 500   200 no nono no no 7, 9 22 500 300-500 no no yes yes yes

Example 10

PBT, Celanex 1600A (Hoechst Celanese), with 0.2% by weight boric acidwas extruded into a tube of inner diameter 0.0170 and outer diameter0.0350 inch. The tube was stretched at room temperature and a speed of12 sec/4 inch until fully necked. The tube was pressurized at 400 psiwhile it was stretched. The stretched tube was then inserted in a 3.00mm balloon mold and a balloon formed at 95° C. with a blowing pressureof 300 psi and a tension of 30 grams.

Balloons prepared in this manner were subjected to standard burst tests.Burst strength and distension are given in Table 2.

Example 11

A stretched tube prepared in accordance with Example 10 was inserted ina 2.50 mm balloon mold and a balloon formed at 98° C. with a blowingpressure of 280 psi and a tension of 30 grams.

Balloons prepared in this manner were subjected to standard burst tests.Burst strength and distension are given in Table 2.

Example 12

A tube prepared in accordance with Example 10 was annealed at 110° C.for 1 hour before stretching and then stretched at a pressure of 420psi. The stretched tube was inserted in a 2.75 mm balloon mold and aballoon formed at 98° C. with a blowing pressure of 350 psi and atension of 30 grams.

Balloons prepared in this manner were subjected to standard burst tests.Burst strength and distension are given in Table 2.

Example 13

A tube of Arnitel EM740 block copolymer (poly(butyleneterephthalate)-block-poly(tetramethylene oxide)) with 0.1% (by weight)boric acid was extruded into a tube of inner diameter 0.0230 and outerdiameter 0.039 inch. The tube was stretched at room temperature with 200psi pressure until fully necked. The stretched tube was inserted into a3.00 mm balloon mold and the balloon formed at 95° C. with a blowingpressure of 300 psi and tension of 40 grams.

Balloons prepared in this manner were subjected to standard burst tests.Burst strength and distension are given in Table 2. TABLE 2 Burst andDistention Test Results of Inventive Balloons Distention DistentionBurst 6 atm-12 atm 6 atm-burst Exam- Double wall pressure (608-1216 kPa)(608 kPa-burst) ple thickness (inch) (psi) (%) (%) 10 0.00125 323 3.79.8 11 0.00150 425 3.1 9.6 12 0.00120 338 3.7 12.1 13 0.00125 279 12.634.8

The process of the invention may also be usefully employed to formballoons of other polymers which have a very high crystallization rate,such as polypropylene, nylon 12, nylon 11, nylon 6, and injectionmolding grades of PET (the latter are typically formulated with anucleating agent to accelerate crystallization).

The above examples and disclosure are intended to be illustrative andnot exhaustive. These examples and description will suggest manyvariations and alternatives to one of ordinary skill in this art. Allthese alternatives and variations are intended to be included within thescope of the attached claims. Those familiar with the art may recognizeother equivalents to the specific embodiments described herein whichequivalents are also intended to be encompassed by the claims attachedhereto.

1. A composition prepared by melt mixing a high butylene terephthalatecontent polymer material with 0.01-5.0% by weight of said composition ofboric acid.
 2. A composition as in claim 1 wherein the polymer materialis a member selected from the group consisting of a) poly(butyleneterephthalate) homopolymer, b) random polyester copolymers having above80% butylene terephthalate repeat units, c) block copolymers comprisingabout 60% or more by weight of poly(butylene terephthalate), d) mixturesof at least two of a), b) and/or c); and e) mixtures of one or more ofa), b) and/or c) with no more than 10% by weight of another polymer. 3.A composition as in claim 1 wherein the polybutylene terephthalatecontent is sufficient to cause opacification of extruded material in theabsence of said boric acid.
 4. A composition as in claim 1 wherein saidpolymer material is poly(butylene terephthalate) homopolymer.
 5. Acomposition as in claim 1 wherein said polymer material is poly(butyleneterephthalate)-block-poly(tetramethylene oxide).
 6. A composition as inclaim 1 wherein the amount of boric acid is from about 0.05 to 0.5% byweight.
 7. A composition as in claim 1 wherein the amount of boric acidis from about 0.1 to 0.3% by weight.